Waste water of high salt content is a common product of various chemical industry. For examples, in the dyestuff factory the typical salt contents in the waste water are as follows:
______________________________________ NaCl (or KCl) ca. 20% COD ca. 100,000 ppm Na.sub.2 SO.sub.4 trace. ______________________________________
In the Epoxy Resin industry, the waste water contains the various salts indicated in the following:
______________________________________ NaCl 20% NaOH 1% NaHPO.sub.4 0.05% Glycerine 0.07-0.15% MIBK 0.7-1.0% Polymer/Resin 0.03-0.05% COD 30,000 ppm. ______________________________________
In the Polycarbonate factory, the salt contents in the waste water are:
______________________________________ NaCl 19% NaOH 25-30 mg/L Fe++ 0.37 mg/L Ca++ 2.0 mg/L Total organics &lt;224 ppm Sodium gluconate &lt;200 ppm Methylene choloride &lt;1 ppm. Triethylamine &lt;0.35 ppm. Phenolics as BPA &lt;0.77 ppm. ______________________________________
To process such waste water of high salt contents, the biological systems are not applicable unless the concentration of salts in the water is lowered. Current method is to recover the salts from the waste water to enable the latter to contain lower concentration of salts and be processable by the biological system. The process of recovering salts form such waste water is divided into two parts, e.g., crystallization and heat treatment. In the process of crystallization, the waste water is concentrated to precipitate the growth of the crystals. When the crystals grow to 300-500 .mu.m, they are separated from the liquid phase. Because the final concentration of the liquid before separating the crystals is very high, the surfaces of the crystals are coated with a layer of organic compounds. With one to two molecule thickness. To rid of these organic compounds, the crystals are further treated with heat. Currently, there are two methods of heat treatment to purify these salt crystals, e.g. direct calcination and hot air. The former directly utilizes flame to calcinate the coated organic compounds. Because its temperature (1.941.degree. C.) exceeds the melting point of sodium chloride ( 800.degree.C.), calcination requires another equipment to recrystallize the molten salts.
In the hot air method, the heat is transferred by convection. And the heat flux of convection q.sub.c, equal to h.sub.c .times..DELTA.T
h.sub.c :heat convection coefficient or film coefficient PA1 .DELTA.T:temperature difference PA1 E:activation energy PA1 T:temperature PA1 Rg:gas constant
Both velocity of hot air wind and temperature difference have a significant effect upon the rate of heat transfer. As a result to consequences can be predicted.
(1) Higher velocity and degree of turbulence decrease the thickness of Laminar Sublayer surrounding the crystals and accelerate heat transfer.
(2) The driving force is proportional to temperature difference.
However, to prevent overheating the crystals, the crystals are heated with hot air to increase the temperature of the system not to exceed a set point such as 600.degree. C. to vaporize or decompose the organics but not melt the crystals. According to Arrhenius equation, ##EQU1## K:specific rate constant A:freuqency factor
It is clear that the higher the temperature, the more efficient the decomposition. However, to avoid the crystals from melting, the temperature of the system is not to exceed certain level. This limitation leads to the suboptimal heat transfer rate and decomposition rate which becomes an inherent drawback of this method.